1. Field of the Invention
The present invention relates to an apparatus and a method for detecting bacteria in a specimen and in particular to an apparatus and a method for detecting Gram-negative or Gram-positive bacteria contained in a specimen. The present invention also relates to an apparatus and a method for detecting Gram-negative and Gram-positive bacteria contained in a specimen.
2. Description of the Related Art
For generally classifying bacteria, the bacteria are first divided roughly into two groups of bacteria i.e. Gram-positive bacteria and Gram-negative bacteria, on the basis of Gram stainability. The classification by Gram stainability is the most fundamental standard for classifying bacteria, and Gram staining classification methods generally used at present include Gram staining methods such as a Hucker modified method and a Bartholomew & Mittwer method (Barmi method). These methods fundamentally require techniques wherein Gram stainability is judged by observing a stained specimen under a microscope.
As techniques of detecting and counting Gram-positive bacteria by using a flow cytometer, on one hand, a method described in U.S. Pat. No. 5,137,810 is known. In this method, lectin is used. The lectin is a sugar-binding protein, and sugar chains to which lectin binds exist on the cell surface of a bacterium. One kind of lectin, wheat germ agglutinin (WGA), has such properties as to bind to sugar chains existing abundantly on the cell surface of Gram-positive bacterium. Accordingly, Gram-positive bacteria can be detected by mixing fluorescence-labeled WGA with bacteria and detecting fluorescence.
As techniques of judging whether bacteria contained in a specimen are Gram-negative or Gram-positive bacteria and counting the bacteria by using a flow cytometer, a method described in U.S. Pat. No. 5,545,535 is known. This is a method of classifying Gram-negative and Gram-positive bacteria by staining bacteria with a reagent containing a plurality of fluorescent dyes different from one another in respect of staining specificity and fluorescence wavelength, and classifying the bacteria from the fluorescence pattern. For example, when SYTO 9 that is a fluorescent dye staining bacteria and hexidium iodide that is a fluorescent dye staining only Gram-positive bacteria are used, it can be judged that bacteria stained with both the dye are Gram-positive bacteria, while bacteria stained with only SYTO 9 are Gram-negative bacteria.
By the method using lectin described in U.S. Pat. No. 5,137,810, however, there are cases where accurate judgment results cannot be obtained depending on the kind of bacterium.
By the method using a plurality of fluorescent dyes described in U.S. Pat. No. 5,545,535, there are cases where accurate judgment results cannot be obtained because contaminants other than bacteria contained in a specimen are also stained.